In order to prevent the fuel vapor which vaporizes in a fuel tank of a vehicle engine from being discharged to the atmosphere, the fuel vapor is led to a canister together with air through a vapor passage connecting the fuel tank and the canister, active carbon in the canister absorbs only fuel particles, and the remaining air is discharged from an atmospheric connection port of the canister.
When predetermined running conditions are met, a purge cut valve provided in a purge passage is opened, the absorbed fuel separates from the active carbon due to fresh air entering the canister from the aforesaid atmospheric connection port as a result of the engine intake negative pressure, mixes with the intake air, and is burnt.
Such a system is known as a fuel vapor purge system.
However, when there is a leak hole from the fuel tank to the engine intake and there is a defect in pipe join seals, fuel escapes to the atmosphere. In this regard, OBDII, which has been an obligatory fault diagnostic function for all U.S. made vehicles since 1994, specifies that if there is a leak hole of 1 mm diameter or more in the passage from the fuel tank to the purge cut valve, a warning lamp must light. In this case, the existence of a leak may be detected by monitoring pressure fluctuations in the fuel vapor flowpath when the flowpath is a closed space having a difference from atmospheric pressure.
U.S. Pat No. 5,542,397 discloses a method where a positive pressure is first introduced into the flowpath to perform leak diagnosis, and when a positive pressure cannot be obtained, the negative intake pressure of the engine is introduced into the flowpath to perform leak diagnosis.
In both cases, the flowpath must be sealed during the diagnosis, and an air conducting valve installed in the atmospheric connection port of the canister is therefore closed.
In such a leak diagnostic device, it may occur that refueling is performed while a diagnosis is being made. To perform refueling, a fuel nozzle is generally inserted in a filler tube connected to the fuel tank. The filler tube is so constructed that the fuel nozzle is in intimate contact with its inner circumference, so that fuel vapor does not escape from the filler tube to the outside. In this case, air from the fuel tank is discharged from the atmospheric connection port of the canister as refueling progresses.
However, if fuel is supplied when the air conducting valve is closed, there is no path from which the air in the fuel tank can escape, hence even when fuel flows in from the filler tube, it does not enter the fuel tank due to the pressure rise in the tank. Such a situation may occur for example when leak diagnosis is started while the vehicle is running, the vehicle is stopped and refueling is performed without stopping the engine while the diagnosis is being performed.